Modeling context in mobile distributed systems with the UML

Context-awareness plays an important role in mobile distributed systems since it enables the adaptation of mobile devices to the users. However, one of the major challenges is the preservation of the users’ privacy. Many different approaches of modeling the context of the user exist, but the incorporation of privacy restrictions into context models, which makes the protection of privacy apparent, is missing. This paper presents the Context Modeling Profile (CMP), a lightweight UML (Unified Modeling Language) extension, as a visual language for context models in mobile distributed systems. The resulting models embody metainformation of the context, i.e. source and validity of context information, and reflect privacy restrictions. The profile provides several well-formedness rules for context models and supports the development of context-aware mobile applications through an adequate visual modeling language. A case study is used to illustrate the approach.     

Design and analysis of a fault tolerant hybrid mobile scheme

Mobile computing systems provide users with access to information regardless of their geographical location. In these systems, Mobile Support Stations (MSSs) play the role of providing reliable and uninterrupted communication and computing facilities to mobile hosts. The failure of a MSS can cause interruption of services provided by the mobile system. Two basic schemes for tolerating the failure of MSSs exist in the literature. The first scheme is based on the principle of checkpointing used in distributed systems. The second scheme is based on state information replication of mobile hosts in a number of secondary support stations. Depending on the replication scheme used, the second approach is further classified as a pessimistic or an optimistic technique. In this paper, we propose a hybrid scheme which combines the pessimistic and the optimistic replication schemes. In the proposed scheme, an attempt is made to strike a balance between the long delay caused by the pessimistic and the high memory requirements of the optimistic schemes. In order to find the best ratio between the number of pessimistic to the number of optimistic secondary stations in the proposed scheme, we used fuzzy logic. We also used simulation to compare the performance of the proposed scheme with those of the optimistic and the pessimistic schemes. Simulation results showed that the proposed scheme performs better than either schemes in terms of delay and memory requirements.     

Programmable mobile networks

Existing mobile systems (e.g., mobile IP, mobile ATM and third generation cellular systems) lack the intrinsic architectural flexibility to deal with the complexity of supporting adaptive mobile applications in wireless and mobile environments. We believe that there is a need to develop alternative network architectures from the existing ones to deal with the demands placed on underlying mobile signalling, adaptation management and wireless transport systems in support of new mobile services, e.g. interactive multimedia and web access. In this paper we present the design, implementation and evaluation of mobiware, a middleware technology that enables the introduction of new services in mobile networks. Mobiware provides a toolkit that service providers can utilize to build services that can dynamically exploit the intrinsic scalable properties of mobile multimedia applications in response to time-varying mobile network conditions. Based on an open programmable networking paradigm, mobiware runs on mobile devices, wireless access points and mobile-capable switch/routers providing a set of open programmable interfaces and distributed objects for adaptive mobile networking. Mobiware is software-intensive and is built on CORBA and Java distributed object technologies. The source code for mobiware v1.0 is freely available (comet.columbia.edu/mobiware) for experimentation.     

The conceptual model of context for mobile commerce applications

Mobile commerce applications adhering to anytime and anywhere paradigm, required to be flexible. They should be able to adapt their interface, services and content towards a certain context. Several proposals for definition of context have been already proposed originating from various areas related to mobile commerce. However, an integrated, formal and methodological approach for the determination and representation of context, adjusted to special characteristics of mobile commerce applications, has not been insofar presented. This is the challenge we address in this paper, through a conceptual model that includes: i) a clear and formal definition of context, ii) the depiction of its specific characteristics as metadata, iii) a methodology for its determination and iv) the presentation of an extension of class diagrams of UML for its representation, all of them tailored to the special nature of mobile commerce applications.     

Introduction to mobile agents

This article will lead you into the world of mobile agents, an emerging technology that makes it much easier to design, implement and maintain distributed systems. You will find that mobile agents reduce network traffic and provide an effective means of overcoming network latency. Perhaps most important, through their ability to operate asynchronously and independently of the process that created them, they help you to construct highly robust and fault-tolerant systems thereby directly or indirectly benefiting the end user.Read on and let us introduce you to software agents, including mobile as well as stationary agents. We will explain the benefits of mobile agents and demonstrate the impact they have on the design of distributed systems. This article then concludes with a brief overview of some contemporary mobile agent systems.This article is based on a chapter of a book by the authors entitledProgramming and Deploying Java ^TM Mobile Agents with Aglets ^TM, ISBN 0-201-32582-9, Addison-Wesley, 1998.     

Context-aware and quality-aware algorithms for efficient mobile object management

The management of positions of mobile objects is an essential prerequisite for many context-aware systems such as advanced traffic management systems or personal assistance systems. In this paper, we present two approaches for the scalable tracking of mobile object trajectories and the efficient processing of continuous spatial range queries, respectively. We show in detail how both approaches utilize the basic concepts of accuracy relaxation and utilization of context information, such as movement predictions, to minimize the number of position updates, the size of trajectory data, and the number of energy-consuming position sensing operations.     

Low-cost checkpointing and failure recovery in mobile computingsystems

A mobile computing system consists of mobile and stationary nodes, connected to each other by a communication network. The presence of mobile nodes in the system places constraints on the permissible energy consumption and available communication bandwidth. To minimize the lost computation during recovery from node failures, periodic collection of a consistent snapshot of the system (checkpoint) is required. Locating mobile nodes contributes to the checkpointing and recovery costs. Synchronous snapshot collection algorithms, designed for static networks, either force every node in the system to take a new local snapshot, or block the underlying computation during snapshot collection. Hence, they are not suitable for mobile computing systems. If nodes take their local checkpoints independently in an uncoordinated manner, each node may have to store multiple local checkpoints in stable storage. This is not suitable for mobile nodes as they have small memory. This paper presents a synchronous snapshot collection algorithm for mobile systems that neither forces every node to take a local snapshot, nor blocks the underlying computation during snapshot collection. If a node initiates snapshot collection, local snapshots of only those nodes that have directly or transitively affected the initiator since their last snapshots need to be taken. We prove that the global snapshot collection terminates within a finite time of its invocation and the collected global snapshot is consistent. We also propose a minimal rollback/recovery algorithm in which the computation at a node is rolled back only if it depends on operations that have been undone due to the failure of node(s). Both the algorithms have low communication and storage overheads and meet the low energy consumption and low bandwidth constraints of mobile computing systems     

GCS-MA: A group communication system for mobile agents

Reliable messaging is a key component necessary for mobile agent systems. Current researches focus on reliable one-to-one message delivery to mobile agents. But how to implement a group communication system for mobile agents remains an open issue, which is a powerful block that facilitates the development of fault-tolerant mobile agent systems. In this paper, we propose a group communication system for mobile agents (GCS-MA), which includes totally ordered multicast and membership management functions. We divide a group of mobile agents into several agent clusters, and each agent cluster consists of all mobile agents residing in the same sub-network and is managed by a special module, named coordinator. Then, all coordinators form a ring-based overlay for interchanging messages between clusters. We present a token-based algorithm, an intra-cluster messaging algorithm and an inter-cluster migration algorithm to achieve atomicity and total ordering properties of multicast messages, by building a membership protocol on top of the clustering and failure detection mechanisms. Performance issues of the proposed system have been analysed through simulations. We also describe the application of the proposed system in the context of the service cooperation middleware (SCM) project.     

An efficient synchronization scheme of multimedia streams in wireless and mobile systems

Recent advances in mobile computing and distributed multimedia systems allow mobile hosts (clients) to access wireless multimedia systems anywhere and at anytime, but not without creating a new set of issues and trade-offs. To the best of our knowledge, there has been very little research in dealing with the synchronization problem in wireless and mobile multimedia systems. In this paper, we propose an efficient distributed synchronization algorithm using quasi-sink for wireless and mobile multimedia systems to ensure and facilitate mobile client access to multimedia objects. We discuss the algorithm and provide its proof of correctness. We also present a set of simulation experiments to evaluate the performance of our scheme using message complexity and buffer usage at each frame arrival time. Our results indicate that our scheme exhibits a significant low message complexity and no underflow and overflow within the bounded delivery time.     

The mobile groups approach for the coordination of mobile agents

We present the concept of mobile groups as a basic mechanism for the reliable coordination of mobile agents. Analogously to traditional group systems, mobile groups also provide message delivery guarantees and virtual synchrony. Furthermore, they make agent mobility not only visible for the group, but also consistently ordered with other group actions (such as crashes, joins, leaves, and other migrations). The mobile groups approach represents a novel mobility support mechanism, which can be used to handle reliability of mobile agents required at both, the application and system level (e.g., for coordinating distributed agents and for reliable agent migration, respectively). In this paper, we discuss the motivations for the mobile groups approach, formally define their properties, and present a membership protocol for such groups. We also discuss some implementation issues and related performance data, and present the advantages of mobile groups against mechanisms commonly employed for the coordination of mobile agents.     

An embeddable mobile agent platform supporting runtime code mobility,interaction and coordination of mobile agents and host systems

Agent technology is emerging as an important concept for the development of distributed complex systems. A number of mobile agent systems have been developed in the last decade. However, most of them were developed to support only Java mobile agents. In order to provide distributed applications with code mobility, this article presents a library, the Mobile-C library, that allows a mobile agent platform, Mobile-C, to be embeddable in an application to support mobile C/C++ codes carried by mobile agents. Mobile-C uses a C/C++ interpreter as its Agent Execution Engine (AEE). Through the Mobile-C library, Mobile-C can be embedded into an application to support mobile C/C++ codes carried by mobile agents. Using mobile C/C++ codes, it is easy to interface a variety of low-level hardware devices and legacy systems. Through the Mobile-C library, Mobile-C can run on heterogeneous platforms with various operating systems. The Mobile-C library has a small footprint to meet the stringent memory capacity for applications in mechatronic and embedded systems. The Mobile-C library contains different categories of Application Programming Interfaces (APIs) in both binary and agent spaces to facilitate the design of mobile agent based applications. In addition, a rich set of existing APIs for the C/C++ interpreter employed as the AEE allows an application to have complete information and control over the mobile C/C++ codes residing in Mobile-C. With the synchronization mechanism provided by the Mobile-C library for both binary and agent spaces, simultaneous processes across both spaces can be coordinated to get correct runtime order and avoid unexpected race condition. The study of performance comparisons indicates that Mobile-C is about two times faster than JADE in agent migration. The application of the Mobile-C library is illustrated by dynamic runtime control of a mobile robot’s behavior using mobile agents.     

基于UML类图模型的一致性检查方法

UML中的类图采用直观的图形化表示方法,有效描述了待建系统的静态特征,为系统设计人员发现系统模型中存在的不一致性和冗余等问题,提供了有效的分析工具。但是对于复杂的系统,完全依靠系统分析人员发现模型中存在的不一致性和冗余等问题是不现实的,应当为建模工具赋以模型自动一致性检查功能。SHOIQ(D)是描述逻辑家族中可判定的子集,它在保证推理可判定的同时,具备较强的描述知识能力。鉴于上述特点,通过从UML类图图元中抽取语义,用SHOIQ(D)形式化描述类图图元,借助自动推理引擎,从而使基于UML类图模型的自动一致性检查功能得到实现。根据该方法改进后的建模工具,可以自动发现基于UML类图模型中存在的不一致性和冗余等问题。     

Replica placement algorithms for mobile transaction systems

In distributed mobile systems, communication cost and disconnections are major concerns. In this paper, we address replica placement issues to achieve improved performance for systems supporting mobile transactions. We focus on handling correlated data objects and disconnections. Frequently, requests and/or transactions issued by mobile clients may access multiple data objects and should be considered together in terms of replica allocation. We discuss the replication cost model for correlated data objects and show that the problem of finding an optimal solution is NP. We further adjust the replication cost model for disconnections. A heuristic "expansion-shrinking" algorithm is developed to efficiently make replica placement decisions. The algorithm obtains near optimal solutions for the correlated data model and yields significant performance gains when disconnection is considered. Experimental studies show that the heuristic expansion-shrinking algorithm significantly outperforms the general frequency-based replication schemes.     

A hands-on look at Java mobile agents

What are the benefits and drawbacks of current Java mobile agent systems? The authors installed and evaluated three leading systems available for download from the Web: General Magic's Odyssey, IBM's Aglets, and ObjectSpace's Voyager-and looked at issues such as ease of installation, feature set, documentation, and cost. We also discuss new capabilities of Java 1.1 that show promise as simple yet powerful means to create mobile agent systems. We conclude with a brief look at the ways in which mobile agents are currently being used and the limitations of today's technologies     

Adaptation in mobile workflow management systems

Workflow management systems (WFMS) are an emerging technology for supporting the coordinated execution of business processes by a group of users. One goal of introducing a WFMS into an enterprise is to integrate all personnel working on a business process into the system. This article describes a new approach to the integration of mobile users into a WFMS. This is of special interest because key personnel such as sales representatives and executives are often travelling and can only be integrated into the WFMS through mobile computing technologies. After introducing an architecture for mobile WFMS, we focus on the handling of one specific feature of mobile systems: the high diversity of environments in which mobile users operate. For handling this feature we have to implement adaptable software systems. In our approach, we start by introducing a formal model of mobile systems. This model offers a basis for the use of optimisation techniques to realise an adaptive mobile WFMS.     

Reasoning about UML/OCL class diagrams using constraint logic programming and formula

Model Driven Engineering promotes the use of models as the main artifacts in software and system development. Verification and validation of models are key activities to ensure the quality of the system under development. This paper presents a framework to reason about the satisfiability of class models described using the Unified Modeling Language (UML). The proposed framework allows us to identify possible design flaws as early as possible in the software development cycle. More specifically, we focus on UML Class Diagrams annotated with Object Constraint Language (OCL) invariants, which are considered to be the main artifacts in Object-Oriented analysis and design for representing the static structure of a system. We use the Constraint Logic programming (CLP) paradigm to reason about UML Class Diagrams modeling foundations. In particular, we use Formula as a model-finding and design space exploration tool. We also present an experimental Eclipse plug-in, which implements our UML model to Formula translation proposal following a Model Driven Architecture (MDA) approach. The proposed framework can be used to reason, validate, and verify UML Class Diagram software designs by checking correctness properties and generating model instances using the model exploration tool Formula.     

Pull-based recommendations in mobile environments

In the field of Context-Aware Recommendation Systems (CARS), only static contextual information is usually considered. However, the dynamic contextual information would very helpful in mobile computing scenarios. Despite this interest, the design and implementation of flexible and generic frameworks to support an easy development of context-aware mobile recommendation systems have been relatively unexplored. In this paper, we describe a framework that facilitates the development of CARS for mobile environments. We mainly focus on the development of the elements needed to support pull-based recommendations and the experimental evaluation of the proposed system.     

Integrating mobile agents into the mobile middleware

Mobile agents are a new paradigm for distributed computing that is especially well suited for mobile computing over global wireless networks. This paper describes the approach taken in the ACTS On TheMove project to integrate a mobile agent system into the Mobile Application Support Environment (MASE), a middleware for mobile computing. In this project, an existing mobile agent system was adapted for the requirements of mobile computing. We present the changes that had to be made to the agent system to adapt it to the wireless communication. We also present some of the application areas where a mobile agent system is suitable for mobile communication. We describe an agent based pre-fetcher application where an agent operaes disconnected from the user on the fixed network and prepares web pages for the anticipated next connection of the user using the Quality-of-Service trading functions available in MASE.     

Advanced mobile and wearable systems

The recent spectacular progress in the microelectronic, information, communication, material and sensor technologies created a big stimulus towards development of smart communicating cyber-physical systems (CPS) and Internet of Things (IoT). CPS and IoT are undergoing an explosive growth to a large degree related to advanced mobile systems like smart automotive and avionic systems, mobile robots and wearable devices. The huge and rapidly developing markets of sophisticated mobile cyber-physical systems represent great opportunities, but these opportunities come with a price of unusual system complexity, as well as, stringent and difficult to satisfy requirements of many modern applications. Specifically, smart cars and various wearable systems to a growing degree involve big instant data from multiple complex sensors or other systems, and are required to provide continuous autonomous service in a long time. In consequence, they demand a guaranteed (ultra-)high performance and/or (ultra-)low energy consumption, while requiring a high reliability, safety and security. To adequately address these demands, sophisticated embedded computing and embedded design technologies are needed. After an introduction to modern mobile systems, this paper discusses the huge heterogeneous area of these systems, and considers serious issues and challenges in their design. Subsequently, it discusses the embedded computing and design technologies needed to adequately address the issues and overcome the challenges in order to satisfy the stringent requirements of the modern mobile systems.     

A state/event-based model-checking approach for the analysis of abstract system properties

We present the UMC framework for the formal analysis of concurrent systems specified by collections of UML state machines. The formal model of a system is given by a doubly labelled transition system, and the logic used to specify its properties is the state-based and event-based logic UCTL. UMC is an on-the-fly analysis framework which allows the user to interactively explore a UML model, to visualize abstract behavioural slices of it and to perform local model checking of UCTL formulae. An automotive scenario from the service-oriented computing (SOC) domain is used as case study to illustrate our approach.